Optical fibre tensioning device and method of controlling the tension applied to an optical fibre

ABSTRACT

Optical fiber tensioning device, receiving at its input an optical fiber ( 35 ) from a traction device ( 37 ) and capable of supplying the optical fiber from its output to a device ( 40 ) for storing the fiber. The tensioning device ( 1 ) comprises at least one fixed pulley ( 32 ) on which the optical fiber ( 35 ) is wound, for a certain length at least, and at least one movable pulley ( 9 ) on which the optical fiber ( 35 ) is wound, for a certain length at least. A device for measuring the tension applied to the fiber and a motorized device ( 24, 26, 5, 7 ) for moving the movable pulley ( 9 ) interact with each other to control the distance (Dp) between the axes ( 33, 20 ) of the pulleys ( 32, 9 ) in such a way as to automatically keep the tension applied to the fiber essentially constant.

DESCRIPTION

[0001] The present invention relates to an optical fibre tensioningdevice and a method of controlling the tension applied to an opticalfibre, particularly during the winding of the fibre.

[0002] Typically, optical fibres are formed in draw towers from preformsof glass material. A draw tower is generally provided with a tractiondevice (“capstan”), for example a motorized pulley, capable of drawingthe optical fibre downwards and feeding it to a storage device; thelatter usually comprises a motorized reel onto which the optical fibreis wound.

[0003] In many operating situations, the speed at which the opticalfibre is drawn by the traction device (the drawing speed) may notcoincide with the speed at which the optical fibre is fed into thestorage device, for example owing to fluctuations of the speed of themotors which drive the traction device and the storage device. Thedifference between these speeds gives rise to variations of tension inthe fibre with respect to a target tension; in particular, a fibretension greater than the target tension may damage the fibre, while atension lower than the target tension may make the storage processinefficient.

[0004] To solve this problem, there is a known practice of usingtensioning devices which are to be interposed between the tractiondevice and the storage device and are capable of controlling the tensionof the optical fibre. In particular, there are known tensioning devicescomprising at least one fixed pulley and one movable pulley which canguide the optical fibre. The movement of the movable pulley enables thetension of the optical fibre to be modified. Here and in the remainderof the description, the term “fixed pulley” denotes a pulley which isfree to rotate about its axis and in which the position of the axis isfixed, while the term “movable pulley” denotes a pulley which is free torotate about its axis and in which the position of the axis is variable.

[0005] U.S. Pat. No. 4,712,866 describes a tensioning device positionedbetween a traction device of a draw tower and a storage devicecomprising a take-up reel. This tensioning device comprises a pair offixed pulleys and a movable pulley located on an oscillating arm, alsocalled a “dancer arm”, which carries a weight. The dancer arm actuates aconventional speed control so that the fibre is wound onto the take-upreel at a tension determined by the aforesaid weight.

[0006] U.S. Pat. No. 5,790,292 relating to an optical fibre transmissionline describes, among other things, a draw tower in which a tensioningdevice with two pulleys is interposed between a traction device and atake-up reel (FIG. 7).

[0007] U.S. Pat. No. 4,138,069 describes a winding device for opticalglass filaments in which a traction device draws the filament from asource of molten glass and feeds it through a tensioning device to a setof take-up reels. The tensioning device comprises two fixed pulleys andone movable pulley mounted on one end of a dancer arm. The dancer armextends from an apparatus in which a spring or other means applies aconstant force to move the movable pulley away from the fixed pulleys.

[0008] Tensioning devices can also be used in apparatus other than drawtowers.

[0009] U.S. Pat. No. 5,076,104 describes an apparatus for measuring theload at failure of an optical fibre, in which tensioning devicescomprising a pair of fixed pulleys and one movable pulley are provided.Also provided is a tensioning device comprising one fixed grooved wheeland one movable grooved wheel; the latter is positioned below the fixedgrooved wheel, is movable in a vertical direction and is connected to apredetermined weight. The optical fibre, which in its path is woundaround both the fixed grooved wheel and the movable grooved wheel, isthus kept at a constant tension.

[0010] U.S. Pat. No. 4,505,222 describes an extrusion coating device foroptical fibres, in which a tensioning device is positioned immediatelyupstream of a take-up reel.

[0011] U.S. Pat. No. 4,127,370 describes an extrusion coating apparatusfor an optical fibre, in which a first, a second and a third pulley arepositioned at the entry to the apparatus, immediately downstream of thefibre take-off reel. The first and second pulley are spaced apart fromeach other and from the take-off reel. The third pulley has no supportand is carried on the optical fibre between the first and second pulley,in such a way that the fibre passes, as a result of the weight of thethird pulley, along a U-shaped path between the first and second pulley.A pair of photocells is used to detect the vertical position of thethird pulley and to control the speed of rotation of the take-off reelin such a way that the vertical position of the third pulley, andtherefore the tension of the fibre, remains essentially constant.

[0012] The applicant has observed that tensioning devices of knowntypes, requiring the use of gravity and/or predetermined elastic actionfor controlling the position of the movable pulley, apply anuncontrollable predetermined force to the movable pulley. Therefore itis not possible to modulate in any way the force applied to the movablepulley in order to modify the response of the tensioning device.Consequently, the tension control obtained by means of the knowndevices, being of the non-modulatable type, might be inadequate in someoperating conditions.

[0013] The applicant has found that the aforesaid problem can be atleast partially overcome by using an optical fibre tensioning device inwhich there is a movable pulley on which the optical fibre runs, adevice for measuring the tension applied to the optical fibre and adevice for moving the movable pulley, capable of controlling theposition of the movable pulley in accordance with the signal generatedby the tension measuring device.

[0014] The tensioning device according to the present invention iscapable of receiving at its input an optical fibre from a fibre guide orreturn element or from a device of another type, for example a tractiondevice or a take-off device, and is capable of supplying the opticalfibre from its output, after having automatically controlled itstension, to a further guide or return element, or to a further device,for example a storage device (particularly a reeling device) for theoptical fibre.

[0015] In greater detail, the tensioning device according to the presentinvention comprises a fixed pulley carried by a supporting structure anda movable pulley carried by a slide which is slidable along arectilinear guide carried by the supporting structure.

[0016] The optical fibre entering the tensioning device is wound aroundthe fixed pulley for a certain length, and passes from here to themovable pulley around which it is wound for a further length. Thedistance between the axes of the fixed pulley and the movable pulley isvariable to modify the length of the portion of fibre which extendsbetween the two pulleys and consequently the tension applied to thefibre. To vary the distance between the axes of the fixed pulley and themovable pulley there are used, as mentioned above, a fibre tensionmeasuring device, for example a load cell associated with the fixedpulley, and a device for moving the slide which can control the distancebetween the axes of the pulleys in accordance with the signal suppliedby the tension measuring device, to automatically keep the tension inthe fibre practically constant. The movement device can comprise anelectric motor and a system for transmitting the motion from the motorto the slide, for example a screw and nut coupling in which the screwextends parallel to the rectilinear guide and is rotationally movable bythe force of the motor and in which the nut is coupled to the slide, ora belt transmission, for example a toothed belt transmission.

[0017] Since it automatically controls the tension of the fibre withoutusing the force of gravity or other predetermined forces, the aforesaidoptical fibre tensioning device can be used to modulate in the desiredway the tension applied to the optical fibre.

[0018] In this way, the movement of the movable pulley is made entirelyindependent of the force of gravity or predetermined elastic forces, andis achieved by the application of a controllable force.

[0019] Thus a tensioning device which automatically adapts itself to anyvariation of the operating conditions is provided.

[0020] In a first aspect, the present invention relates to an opticalfibre tensioning device, for use in an optical fibre advance path,comprising at least one movable pulley around which the optical fibre iswound for a certain length at least; the position of the said movablepulley being variable to modify the tension applied to the opticalfibre,

[0021] and additionally comprising a tension measuring device capable ofmeasuring the said applied tension and of generating a signal indicatingthe tension, and a motorized device for moving the said movable pulley,capable of receiving the said signal indicating the tension and ofmoving the movable pulley in accordance with the said signal indicatingthe tension, to automatically keep the tension applied to the opticalfibre essentially constant.

[0022] Preferably, the tensioning device also comprises a fixed pulleyaround which the optical fibre is wound for a certain length at least,the distance between the axes of the said fixed pulley and the saidmovable pulley being variable to modify the tension applied to theoptical fibre, and the said motorized movement device being capable ofcontrolling the said distance to automatically keep the tension appliedto the optical fibre essentially constant.

[0023] The said motorized movement device may comprise a guide alongwhich a slide supporting the said movable pulley can be moved and anactuator device for moving the said slide along the said guide.

[0024] The said actuator device may comprise a screw movablerotationally by the force of an electric motor and a nut connected tothe said slide, or an electric motor and a motion transmission system ofthe belt type (for example, one using a toothed belt) to connect themotor to the slide.

[0025] The tensioning device may comprise an electronic control devicewhich receives reference signals and measurement signals at its input,and is capable of generating, in response to the said input signals,operating signals for the closed-loop control of the said motorizedmovement device.

[0026] The said electronic control device is preferably a device for theclosed-loop control of the tension applied to the optical fibre,comprising:

[0027] a subtractor to which is supplied a tension reference signal,proportional to a target tension of the optical fibre, and a measurementsignal, consisting of the said signal indicating the tension of theoptical fibre; and

[0028] a controller, preferably a proportional-integral derivativecontroller, receiving at its input a tension error signal supplied fromthe output of the said subtractor, and generating at its output a drivesignal which can be used to operate the said motorized movement deviceand to control the said distance.

[0029] Thus a closed-loop control of the tension present in the fibre isprovided, by means of which this tension is constantly monitored,causing an immediate intervention of the movement device whenever themeasured tension deviates from the reference tension. The tension in thefibre is thus kept constantly equal to the reference tension, providinga particularly precise control.

[0030] Advantageously, the said tension measuring device may comprise aload cell connected to the said fixed pulley and capable of generatingat its output the said signal indicating the tension.

[0031] Preferably, the said motorized movement device comprises a guidealong which a slide supporting the said movable pulley can be moved, andan actuator device for moving the said slide along the said guide; thesaid electronic control device implementing a system for the closed-loopcontrol of the position of the said slide along the said guide.

[0032] Preferably, the tensioning device is connected for operation to adevice for storing the optical fibre, capable of receiving the opticalfibre from the tensioning device, and the said system for theclosed-loop control of the position of the said slide along the saidguide comprises:

[0033] a further subtractor to which is supplied a position referencesignal, correlated with a reference position of the said slide along thesaid guide, and a measurement signal, proportional to the actualposition of the said slide along the said guide; and

[0034] a further controller, preferably a proportional-integralderivative controller, receiving at its input a position error signalsupplied from the output of the said further subtractor, and generatingat its output a further drive signal which can be used to operate thesaid storage device.

[0035] The said storage device preferably comprises at least onemotorized reel on which the optical fibre from the said tensioningdevice is wound, the said motorized reel being rotationally movableabout an axis of rotation under the action of an electric motor, and thesaid further drive signal being usable for the control of the saidelectric motor.

[0036] The said electronic control device preferably comprises anintegrator receiving at its input a signal correlated with the speed ofmovement of the said slide along the said guide and generating at itsoutput the said measurement signal proportional to the actual positionof the said slide along the said guide.

[0037] In this way, it is ensured that the movable pulley is always keptin the proximity of a reference position from which it is alwayspossible to move the movable pulley in a useful way to compensate for avariation of tension; thus the slide is not permitted to be positionedat a terminal position at which it is no longer possible to move themovable pulley in one of the directions of movement.

[0038] In a further aspect, the present invention relates to a method ofcontrolling the tension applied to an optical fibre along a path ofadvance of an optical fibre, the said method comprising the stage offorming a portion of the said path having a length which can be variedto modify the tension applied to the said fibre, and the stages ofmeasuring the tension applied to the optical fibre and controlling thesaid length in an automatic way, by means of a motor, in accordance withthe said measured tension, to keep the tension of the optical fibrepractically constant.

[0039] Preferably, the method comprises the stage of closed-loop controlof the tension applied to the said optical fibre in accordance with atleast one reference signal, proportional to a target tension of thefibre, and a measurement signal which is generated in this stage ofmeasuring the tension and is proportional to the tension to which theoptical fibre is actually subjected.

[0040] The said stage of closed-loop control of the tension preferablycomprises the stages of:

[0041] comparing the said tension reference signal with the saidmeasurement signal, generating a tension error signal; and

[0042] processing the said error signal, generating at the output adrive signal which can be used to modify the said length by means of amotor and to keep the tension in the fibre essentially constant.

[0043] The method preferably comprises the stage of closed-loop controlof the position of a movable member in accordance with at least oneposition reference signal which is proportional to a target position ofthe said movable member, and a measurement signal which is proportionalto the instantaneous position of the said movable member, the variationof the position of the said movable member causing the variation of thesaid length.

[0044] The method preferably also comprises the stage of storing theoptical fibre at the end of the said advance path; the said stage ofclosed-loop control of the position of the said movable member comprisesthe stages of:

[0045] comparing the said position reference signal with the saidposition measurement signal, generating a position error signal; and

[0046] processing the said position error signal, generating at theoutput a drive signal which can be used to modify the speed at which theoptical fibre is stored.

[0047] In a further aspect, the present invention relates to an opticalfibre processing system, for example a draw tower, an extrusion line oran apparatus for measuring the breaking strength of an optical fibre, inwhich there is an optical fibre tensioning device capable of receivingat its input an optical fibre from a source of optical fibre, forexample an optical fibre traction device, and capable of supplying theoptical fibre from its output to a device for storing the optical fibre,for example a motorized reel, the said tensioning device comprising atleast one movable pulley around which the said optical fibre is woundfor a certain length at least, the position of the said movable pulleybeing variable to modify the tension applied to the said fibre, thetensioning device additionally comprising a tension measuring devicecapable of measuring the said applied tension and of generating a signalindicating the tension, and a device for the motorized movement of thesaid movable pulley which is capable of receiving the said signalindicating the tension and of moving the movable pulley in accordancewith the said signal indicating the tension to automatically keep thetension in the fibre practically constant.

[0048] Preferably, the system comprises an electronic control devicecapable of providing a system of closed-loop control of the tensionapplied to the optical fibre, and comprising:

[0049] a subtractor to which is supplied a tension reference signal,proportional to a target tension of the optical fibre, and a measurementsignal, consisting of the said signal indicating the tension; and

[0050] a controller, preferably a proportional-integral derivativecontroller, receiving at its input a tension error signal supplied fromthe output of the said subtractor, and generating at its output a drivesignal which can be used to operate the said motorized movement device.

[0051] Preferably, the said motorized movement device comprises a guide,along which a slide supporting the said movable pulley can be moved, andan actuator device for moving the said slide along the said guide; thesaid electronic control device providing a system of closed-loop controlof the position of the said slide along the said guide.

[0052] Preferably, the tensioning device is connected for operation tothe optical fibre storage device, and the said system for theclosed-loop control of the position of the said slide along the saidguide comprises:

[0053] a further subtractor to which is supplied a position referencesignal, correlated with a reference position of the said slide along thesaid guide, and a measurement signal, proportional to the actualposition of the said slide along the said guide; and

[0054] a further controller, preferably a proportional-integralderivative controller, receiving at its input a position error signalsupplied from the output of the said further subtractor, and generatingat its output a further drive signal which can be used to operate thesaid storage device.

[0055] The invention will now be illustrated with particular referenceto the attached drawings which represent, without restriction, apreferred embodiment of the invention in which:

[0056]FIG. 1 shows, in plan view, a tensioning device for opticalfibres, made according to the present invention; and

[0057]FIG. 2 shows a block diagram of an electronic unit for controllingthe device of FIG. 1, made according to the present invention.

[0058] In FIG. 1, the number 1 indicates, as a whole, an optical fibretensioning device.

[0059] The tensioning device 1 comprises a supporting panel 3 (made frommetal, for example) carried by a frame (not illustrated) which allows itto be positioned in a fixed way, for example in a horizontal or avertical position.

[0060] The tensioning device 1 also comprises at least one rectilinearguide 5, which in this particular case is formed by two cylindrical rods12 a, 12 b carried by the supporting panel 3, and a slide 7 movable witha reversible motion along the rectilinear guide 5 and supporting a firstpulley 9, referred to hereafter as the movable pulley. The direction ofmovement of the slide 7 is indicated by D in FIG. 1.

[0061] The cylindrical rods 12 a, 12 b are positioned parallel to eachother, and have their respective end portions carried by pairs ofsupports 15 a, 15 b fixed to the supporting panel 3. The slide 7 canhave a rectangular shape in plan view, and can be provided with slidingelements 7 a, 7 b engaged with the rods 12 a, 12 b respectively; forexample, the slidable engagement between the slide 7 and the rods 12 a,12 b can be formed by means of sliding bearings 18, or ball-bearingcollars.

[0062] The movable pulley 9 is preferably carried by a pin 20 whichextends from, and is perpendicular to, the slide 7, in such a way thatthe axis of rotation of the movable pulley 9 is perpendicular to thedirection D of movement of the slide 7.

[0063] The slide 7 can be moved by the force of an electrical motor 24,preferably a motor having an output shaft connected to a screw 26located between the rods 12 a, 12 b and extending parallel to these. Themotor 24 can be mounted in a projecting way on a bracket 28 fixed to thepanel 3, and the ends of the screw 26 are preferably supported bycorresponding supports 30 a, 30 b which are also fixed to the panel 3.Advantageously, the screw 26 is coupled to a circulating ball nut (notillustrated) fixed to the slide 7 on the side opposite the movablepulley 9.

[0064] Alternatively, the transmission of motion from the motor 24 tothe movable pulley 9 could be provided by means of a belt transmission,with a toothed belt for example (not illustrated).

[0065] The tensioning device 1 also comprises a second pulley 32,referred to hereafter as the fixed pulley, carried by a pin 33 whichpreferably extends perpendicularly to the panel 3, for example in aportion of the panel 3 opposite that occupied by the motor 24.

[0066] The fixed pulley 32 preferably has the same diameter as themovable pulley 9, and the axes of rotation of the pulleys 9 and 32 arepreferably parallel to each other.

[0067] The angular rotation of the electric motor 24 produces the linearmovement of the slide 7 along the guide 5, thus modifying the distanceDp between the axes of rotation of the movable pulley 9 and the fixedpulley 32.

[0068] In particular, the distance Dp is continuously variable between aminimum value (DpMin) for which the slide 7 is located at a firstterminal position (not illustrated) in which it bears on the brackets 15a, 15 b which are farther from the motor 24, and a maximum value (DpMax)for which the slide 7 is located at a second terminal position (notshown) at which it bears on the brackets 15 a, 15 b which are closer tothe motor 24.

[0069] The tensioning device 1 can also comprise a transparentprotective enclosure (carter—not shown) which can be connected to thesupporting panel 3 and can house all the parts carried by the panel 3.

[0070] The tensioning device 1 is capable of being fitted in an advancepath of an optical fibre 35, for example in a draw tower (notillustrated) for the optical fibre, and is capable of receiving theoptical fibre from a source of optical fibre 37 (shown schematically).In the non-restrictive example considered here, the source of opticalfibre 37 is a traction device (“capstan”) of an optical fibre draw tower(not illustrated). Alternatively, the source of optical fibre 37 couldbe any member or device for returning or guiding the optical fibre, oran optical fibre take-off reel.

[0071] The optical fibre 35 extends from the traction device 37 to thefixed pulley 32, forming a first rectilinear portion 35 a, is wound(through 180° for example) around the fixed pulley 32, extends betweenthe fixed pulley 32 and the movable pulley 9, forming a secondrectilinear portion 35 b, is wound (through 180° for example) around themovable pulley 9, and finally extends between the movable pulley 9 and afurther device 40, thus forming a third rectilinear portion 35 c. In thenon-restrictive example considered here, the device 40 (shownschematically) is a storage device comprising a motorized reel 42 on towhich the optical fibre 35 can be wound. The motorized reel 42 ismovable rotationally about an axis of rotation 43 under the action of anelectric motor 45. Alternatively, the device 40 can be any member ordevice for returning or guiding the optical fibre.

[0072] Preferably, but not necessarily, the rectilinear portions 35 a,35 b, 35 c are parallel to each other.

[0073] Between the device 37 and the tensioning device 1, and betweenthe tensioning device 1 and the device 40, there may advantageously bereturn pulleys which, for simplicity, are not shown in FIG. 1.

[0074] An electronic control unit 50 is capable of controlling, throughcorresponding power driver circuits (“drivers”) 24 p, 45 p, the electricmotor 24 and the electric motor 45.

[0075] Advantageously, the tensioning device 1 comprises a tensionmeasuring device 56, for example a load cell, preferably connected tothe pin 33 of the fixed pulley 32. The tension measuring device 56 iscapable of generating a signal Tmis proportional to the tension to whichthe optical fibre 35 is subjected, and is connected to the electroniccontrol unit 50 to supply the signal Tmis to the electronic control unit50.

[0076] With particular reference to FIG. 2, the electronic unit 50 isillustrated in detail. The electronic unit 50 comprises a subtractor 60to which is supplied a signal T₀, proportional to a target tension ofthe fibre, and the signal Tmis, proportional to the tension to which theoptical fibre 35 is actually subjected. The subtractor 60 generates atits output an error signal ε=ΔT=T₀−Tmis (tension error) and suppliesthis error signal to a controller 62, particularly a PID(proportional-integral derivative) controller, which generates at itsoutput a signal ω (a voltage signal, compatible with the drive circuit24 p which is operated by means of voltage).

[0077] A PID controller is a controller which, having received an inputsignal S_(i), generates an output signal S_(o), typically given by:${S_{o}(t)} = {K_{p} \cdot {\left( {{S_{\iota}(t)} + {\frac{1}{T_{\iota}}{\int_{0}^{\iota}{{S_{\iota}(t)} \cdot {t}}}} + {T_{d} \cdot \frac{{S_{\iota}(t)}}{t}}} \right).}}$

[0078] A PID controller can therefore be identified by the threeconstants K_(p), K_(i)=K_(p)/T_(i) and K_(d)=K_(p)·T_(d).

[0079] The signal ω is used, after having been converted and transformedinto a power signal by the drive circuit 24 p (of a known type), tooperate the motor 24. The speed of rotation of the motor 24 is thereforecorrelated with the signal ω. The signal ω is also supplied to aconversion unit 64, which generates at its output a signal V correlatedwith the linear speed of movement of the slide 7 along the guide 5.

[0080] In practice, if v_(max) is the maximum speed of the motor 24(expressed in r.p.s.), ω_(max) is the (electric voltage) value of thesignal ω with which the maximum speed v_(max) is associated, p is thepitch of the screw 26 and ω(t) is the generic value of the signal ω atthe instant t (supplied from the output of the controller 62), theconversion unit 64 generates at its output a signal V whose value V(t)at the instant t is given by:${V(t)} = {\frac{v_{\max}}{\omega_{\max}} \cdot {\omega (t)} \cdot p}$

[0081] In practice, the conversion unit 64 implements the transferfunction which relates the angular rotation of the screw 26 to thelinear speed of the slide 7.

[0082] The signal V is supplied to a further conversion unit 66, which,by integrating the signal V, generates at its output a signal Pmis(electric voltage) relating to the instantaneous position of the slide 7along the guide 5.

[0083] The position signal Pmis is supplied to a subtractor 70 to whicha signal P₀, proportional to a target position of the slide 7 along theguide 5, is also supplied. Preferably, the target position of the slideis the position intermediate between the first and second terminalpositions.

[0084] The subtractor 70 generates at its output an error signalε=ΔP=P₀−P_(mis), and supplies this error signal to a controller 72 (inparticular, a PID (proportional-integral derivative) controller), whichgenerates at its output a signal ωb which is used, after having beenconverted and transformed into a power signal by the drive circuit 45 p(of a known type), to operate the motor 45. The signal ωb is correlatedwith the speed of rotation of the reel 42.

[0085] In operation, when the storage device imparts to the incomingoptical fibre 35 a higher speed than the speed at which it is suppliedfrom the output of the traction device 37, the tension Tmis in the fibreincreases and deviates from the target value T₀.

[0086] Consequently, the tension error ΔT (which is negative in thiscase) increases in absolute value, and the controller 62, through thedrive circuit 24 p, causes a rotation of the motor 24 such that theslide 7 is made to move towards the first terminal position, andconsequently the distance Dp and the length of the portion 35 b of fibrelocated between the two pulleys 9 and 32 are decreased; the decrease ofthe length of the portion 35 b is thus opposed to the increase oftension in the fibre, in such a way that the tension in the fibre iskept essentially constant.

[0087] Conversely, when the storage device 40 imparts to the incomingoptical fibre 35 a lower speed than the speed at which it is suppliedfrom the output of the traction device 37, the tension Tmis in the fibredecreases and deviates from the target value T₀. Consequently, thetension error ΔT (which is positive this time) increases, and thecontroller 62, through the drive circuit 24 p, causes a rotation of themotor 24 such that the slide 7 is made to move towards the secondterminal position, and consequently the distance Dp and the length ofthe portion 35 b are increased; the increase of the length of theportion 35 b is opposed to the decrease of tension in the fibre in sucha way that the tension in the fibre is kept essentially constant.

[0088] Thus a motorized closed-loop control of tension is provided,which returns the tension in the fibre to the target value T₀.

[0089] Additionally, the unit 66, by integrating the signal V(correlated with the linear speed of the slide 7), determines theinstantaneous position Pmis of the slide 7 along the guide 5. Thisinstantaneous position is compared with the objective position P₀, andthe position error ΔP, processed by the conversion unit 72, produces avariation of the speed of rotation of the motor 45. In particular, whenthe slide 7 is closer than the reference position to the first terminalposition (with the slide 7 moved to the left in FIG. 1), the controlsystem produces a decrease of the speed of rotation of the motor 45 insuch a way that the tension in the fibre 35 decreases and the tensioncontrol loop, in order to compensate for this variation of tension,moves the slide 7 (by means of the motor 24 and the screw 26) towardsthe second terminal position (in other words to the right in FIG. 1),thus compensating for the variation of position.

[0090] Additionally, when the slide 7 is closer than the referenceposition to the second terminal position (with the slide 7 moved to theright in FIG. 1), the control system produces an increase of the speedof rotation of the motor 45 in such a way that the tension in the fibre35 increases and the tension control loop, in order to compensate forthis variation of tension, moves the slide 7 (by means of the motor 24and the screw 26) towards the first terminal position (in other words tothe left in FIG. 1), thus compensating for the variation of position.

[0091] Thus a closed-loop position control is provided, which returnsthe slide to the target position P₀.

[0092] If preferred, the position controller could be dispensed with; inthis case, the control device would not include the elements indicatedby the numbers 64, 66, 70, 72 and 45 p.

[0093] The above description clearly shows the advantages of the presentinvention, in that the movement of the movable pulley 9 depends solelyon the action of the electric motor 24 and is entirely independent ofthe force of gravity or other predetermined forces (of the elastic type,for example) applied to the movable pulley 9. This, a force whose valuecan be automatically controlled in accordance with the operating signalapplied to the motor 24 is applied to the optical fibre 35. Thetensioning device 1 adapts itself automatically to any change inoperating requirements (such as a change in the drawing speed, or achange in the position of the slide 7) and returns the value of thetension applied to the fibre to a target value.

[0094] Additionally, it is ensured that the movable pulley 9 is alwayskept close to a reference position (in particular, the central positionof the guide 5) from which it can always be moved in a useful way (tothe right or to the left) to effectively oppose a change in tension. Inother words, the slide 7 is prevented from being positioned close to thefirst or second terminal position, where the movement of the slide inone of the two directions is no longer possible.

[0095] The tensioning device 1 can be used in apparatus of differenttypes, for example in a draw tower, in an extrusion line or in a testapparatus (also known as a “screening apparatus”) for testing theoptical fibre in respect of the breaking stresses.

[0096] A draw tower typically comprises a furnace for melting a preformof glass material, a traction member (or “capstan”, for example one ofthe single pulley or double pulley type) for drawing downwards theoptical fibre generated from the melted preform, a coating devicepositioned between the furnace and the traction member to apply aprotective surface coating (typically made from acrylate) to the opticalfibre, and a fibre take-up reel receiving the fibre from the tractionmember. The tensioning device 1 can advantageously be positioned, asmentioned above, between the traction member and the take-up reel.

[0097] An extrusion line is a processing line along which a plurality ofoptical fibres are made to advance with a wire-shaped supportingelement, and along which a polymer material is extruded over thesupporting element in such a way as to incorporate the optical fibresand hold them in a fixed position around the supporting element, thusforming the optical core of an optical fable. An extrusion linetypically comprises a take-off reel for the supporting element, aplurality of fibre take-off reels, an extruder capable of receiving boththe fibres and the supporting element at its inlet, a cooling vesselreceiving from the extruder the optical core formed in the extruder, anda final take-up reel. In an extrusion line, the tensioning device 1 can,for example, be positioned downstream of each of the fibre take-offreels.

[0098] A screening apparatus is typically used to check that the fibreis capable of withstanding predetermined tensions, and is therefore freeof significant structural defects. A screen apparatus typicallycomprises a take-off reel for the optical fibre, a take-up reel for theoptical fibre and a plurality of guide and/or return elements capable offorming a path for the optical fibre between the take-off and take-upreels and capable of imparting a predetermined tension to the opticalfibre during its passage. In a screening apparatus, the device 1 can bepositioned at any point of the aforesaid path.

[0099] Finally, it is clear that modifications and variations can bemade to the tensioning device described without departing from the scopeof protection of the present invention.

[0100] In particular, the system for moving the slide 7 (screw 26 andnut coupled to the slide 7) could be different from that described, andcould comprise, for example, a toothed belt (not illustrated) connectedto the slide 7 and movable between two end pulleys.

[0101] Finally, the position Pmis of the slide 7 along the guide 5 couldbe determined not in an indirect way (in other words, following themeasurement of the speed) as illustrated in FIG. 2, but in a direct way,using a row of position sensors (not illustrated), for example a row ofphotocells, or an optical strip (in other words a linear photosensitivecomponent, for example a linear CCD—not illustrated), arranged along theguide 5.

1. Optical fibre tensioning device, for use in a path of advance of anoptical fibre (35), comprising at least one movable pulley (9) aroundwhich the optical fibre (35) is wound for a certain length at least; theposition of the said movable pulley (9) being variable to modify thetension applied to the optical fibre (35), characterized in that itcomprises a tension measuring device (56) capable of measuring the saidapplied tension and of generating a signal indicating the tension(Tmis), and a motorized movement device (24, 26, 5, 7, 50) for the saidmovable pulley (9), capable of receiving the said signal indicating thetension (Tmis) and of moving the movable pulley in accordance with thesaid signal indicating the tension (Tmis), to automatically keep thetension applied to the optical fibre (35) essentially constant: 2.Device according to claim 1 , characterized in that it also comprises afixed pulley (32) around which the optical fibre (35) is wound for acertain length at least; the distance (Dp) between the axes (33, 20) ofthe said fixed pulley (32) and of the said movable pulley being variableto modify the tension applied to the optical fibre; the said motorizedmovement device (24, 26, 5, 7) being capable of controlling the saiddistance (Dp) to automatically keep the tension applied to the opticalfibre essentially constant.
 3. Device according to claim 1 or 2 ,characterized in that the said motorized movement device (24, 26, 5, 7)comprises a guide (5) along which a slide (7) supporting the saidmovable pulley (9) can be moved, and an actuator device (26, 24) formoving the said slide (7) along the said guide (5).
 4. Device accordingto claim 3 , characterized in that the said actuator device comprises ascrew (26) which is rotationally movable under the force of an electricmotor (24) and a nut coupled to the said slide (7).
 5. Device accordingto claim 3 , characterized in that the said actuator device comprises anelectric motor (24) and a motion transmission system of the belt type.6. Device according to claim 2 , characterized in that it comprises anelectronic control device (50) which receives reference signals (T₀, P₀)and measurement signals (Tmis, Pmis) at its input, and is capable ofgenerating, in response to the said input signals, operating signals (24p, 45 p) for the closed-loop control of the said motorized movementdevice (24, 26, 5, 7).
 7. Device according to claim 6 , characterized inthat the said electronic control device (50) forms a system for theclosed-loop control of the tension applied to the said optical fibre(35), comprising: a subtractor (60) to which is supplied a tensionreference signal (T₀), proportional to a target tension of the opticalfibre, and a measurement signal (Tmis), consisting of the said signalindicating the tension; and a controller (62), receiving at its input atension error signal (ε=ΔT=T₀−Tmis) supplied from the output of the saidsubtractor (60), and generating at its output a drive signal (ω) whichcan be used to operate the said motorized movement device (24, 26, 5, 7)and to control the said distance (Dp).
 8. Device according to claim 7 ,characterized in that the said controller (62) comprises aproportional-integral derivative controller.
 9. Device according toclaim 7 , characterized in that the said tension measuring device (56)comprises a load cell (56) connected to the said fixed pulley (32) andcapable of generating at its output the said signal indicating thetension (Tmis).
 10. Device according to claim 6 or 7 , characterized inthat the said motorized movement device (24, 26, 5, 7) comprises a guide(5) along which a slide (7) supporting the said movable pulley (9) canbe moved, and an actuator device (26, 24) for moving the said slide (7)along the said guide (5); the said electronic control device (50)implementing a system for the closed-loop control of the position of thesaid slide (7) along the said guide (5).
 11. Device according to claim10 , characterized in that it is connected for operation to a device(40) for storing the optical fibre, capable of receiving the opticalfibre from the tensioning device, and in that the said system for theclosed-loop control of the position of the said slide (7) along the saidguide (5) comprises: a further subtractor (70) to which is supplied aposition reference signal (P₀), correlated with a reference position ofthe said slide (7) along the said guide (5), and a measurement signal(Pmis), proportional to the actual position of the said slide (7) alongthe said guide (5); and a further controller (72), receiving at itsinput a position error signal (ε=ΔP=P₀−Pmis) supplied from the output ofthe said further subtractor (70), and generating at its output a furtherdrive signal (ωb) which can be used (45 p) to operate the said storagedevice (40).
 12. Device according to claim 11 , characterized in thatthe said further controller (72) comprises a proportional-integralderivative controller.
 13. Device according to claim 11 , characterizedin that the said storage device (40) comprises at least one motorizedreel (42) on which the optical fibre (35) from the said tensioningdevice (1) is wound, the said motorized reel (42) being rotationallymovable about an axis of rotation (43) under the action of an electricmotor (45), and the said further drive signal (ωb) being usable (45 p)for the control of the said electric motor (45).
 14. Device according toclaim 11 , characterized in that the said electronic control device (50)comprises an integrator (66) receiving at its input a signal (V)correlated with the speed of movement of the said slide (7) along thesaid guide (5) and generating at its output the said measurement signal(Pmis) proportional to the actual position of the said slide (7) alongthe said guide (5).
 15. Method of controlling the tension applied to anoptical fibre along a path of advance of an optical fibre (35), the saidmethod comprising the stage of forming a portion (35 b) of the said pathhaving a length (Dp) which can be varied to modify the tension appliedto the said fibre, characterized in that it comprises the stage ofmeasuring (56) the tension applied to the optical fibre and the stage ofcontrolling the said length (Dp) in an automatic way, by means of amotor, in accordance with the said measured tension, to keep the tensionapplied to the optical fibre practically constant.
 16. Method accordingto claim 15 , characterized in that it comprises the stage ofclosed-loop control of the tension applied to the said optical fibre inaccordance with at least one reference signal (T₀), proportional to atarget tension of the fibre, and a measurement signal (Tmis) which isgenerated in the said stage of measuring the tension and is proportionalto the tension to which the optical fibre is actually subjected. 17.Method according to claim 16 , characterized in that the said stage ofclosed-loop control of the tension comprises the stages of: comparing(60) the said tension reference signal (T₀) with the said measurementsignal (Tmis), generating a tension error signal (ε=ΔT=T₀−Tmis); andprocessing (62) the said error signal, generating at the output a drivesignal (ω) which can be used (24 p) to modify the said length (Dp) bymeans of a motor and to keep the tension in the fibre essentiallyconstant.
 18. Method according to claim 15 , characterized in that itcomprises the stage of closed-loop control of the position of a movablemember (7) in accordance with at least one position reference signal(P₀) which is proportional to a target position of the said movablemember, and a measurement signal (Pmis) which is proportional to theinstantaneous position of the said movable member, the variation of theposition of the said movable member (7) causing the variation of thesaid length (Dp).
 19. Method according to claim 18 , characterized inthat the optical fibre is stored at the end of the said advance path,and in that the said stage of closed-loop control of the position of thesaid movable member comprises the stages of: comparing (70) the saidposition reference signal (P₀) with the said position measurement signal(Pmis), generating a position error signal (ε=ΔP=P₀−Pmis); andprocessing (72) the said position error signal, generating at the outputa drive signal (ωb) which can be used (45 p) to modify the speed atwhich the optical fibre is stored.
 20. Optical fibre processing system,comprising a source of optical fibre (37), an optical fibre tensioningdevice (1) capable of receiving at its input the optical fibre (35) fromthe source of optical fibre (37), and an optical fibre storage device(40) capable of receiving the optical fibre from the tensioning device(1), the said tensioning device (1) comprising at least one movablepulley (9) around which the optical fibre (35) is wound, for a certainlength at least; the position of the said movable pulley (9) beingvariable to modify the tension applied to the said optical fibre; thesaid system being characterized in that the said tensioning device (1)comprises a tension measuring device (56) capable of measuring the saidapplied tension and of generating a signal indicating the tension(Tmis), and a motorized movement device (7, 24, 26) capable of receivingthe said signal indicating the tension and of moving the movable pulley(9) in accordance with the said signal indicating the tension, toautomatically keep the tension in the fibre essentially constant.